Control of recovery boiler operation by IR spectroscopy

ABSTRACT

A direct monitoring and control method is provided for on-line measurement of effective alkali, carbonate, sulfate and thiosulfate concentrations in process liquors for the production of kraft pulp. The control method eliminates frequent sampling, and the need for frequent equipment maintenance. The method includes the steps of withdrawing samples of a liquor from the kraft manufacturing process, subjecting the samples to infrared spectrophotometry at predetermined wave numbers to produce peak-absorbance measurements relative to a background spectrum of water, determining peak absorbance for different alkali concentrations, correlating relationships between the peak-absorbance measurements of samples with the peak absorbance for different alkali concentrations to determine optimum effective alkali in the samples, and controlling at least one process parameter to obtain effective alkali of the liquor.

This application is a divisional of U.S. Ser. No. 07/910,379 filed onJul. 8, 1992 and issued as U.S. Pat. No. 5,282,931.

TECHNICAL FIELD

The present invention relates to kraft pulp manufacturing processes andmore specifically to a rapid method for determining and controllingeffective alkali and inorganic salt concentrations during the digesting,recausticizing and recovery operations of a kraft mill.

BACKGROUND ART

Kraft pulping is performed by cooking wood chips in a highly alkalineliquor which selectively dissolves lignin and releases the cellulosicfibers from the wooden matrix. The two major chemicals in the liquor aresodium hydroxide and sodium sulfide. Sodium sulfide, also a strongalkali, readily hydrolyses in water producing sodium hydroxide. Thesulfidity is the amount of sodium sulfide in solution divided by thetotal amount of sodium sulfide and sodium hydroxide. The sulfidity isusually expressed as a percentage which varies between 20 and 30% inpulping liquors. The total amount of sodium hydroxide in solution iscalled effective alkali (EA) before pulping or residual effective alkali(REA) after pulping. Timely knowledge of the REA ensures good control ofthe pulping process.

At the beginning of the kraft process white liquor is fed to thedigester. This liquor contains a high amount of effective alkali. At theexit of the digester the spent liquor or black liquor is extracted fromthe digester. This black liquor contains low levels of effective alkali.Black liquor also contains large amounts of organic compounds which areburned in a recovery furnace. The mass of inorganic residues, calledsmelt, is then dissolved to form green liquor having a low concentrationof effective alkali and a high concentration of sodium carbonate. Whiteliquor is then regenerated from the green liquor by causticizing thecarbonate through the addition of lime. After the recausticizingoperation, a small residual amount of sodium carbonate is carried overto the digester. The total amount of sodium hydroxide, sodium sulfideand sodium carbonate is called the total titratable alkali. Thecausticizing efficiency (CE) is usually defined as the difference in theamounts of sodium hydroxide between the white and green liquors dividedby the amount of sodium carbonate in the green liquor. Sodium sulfateand sodium thiosulfate, together with sodium carbonate, represent a deadload in the liquor recycling system. Sodium thiosulfate is particularlyundesirable in processed liquors because of the potential for corrosionof metal surfaces in contact with these liquors. The reductionefficiency (RE) is defined as the amount of green liquor sodium sulfide,divided by the combined amounts of sodium sulfide and sodium sulfate ineither green liquor or the smelt. A reduction in dead load chemicals hasa beneficial impact on kraft mill operations, thus there is a need forbetter control of all aspects of kraft mill operations and moreefficient use of all chemicals involved in the process. The timelyknowledge of the white liquor charge of effective alkali and blackliquor charge of residual effective alkali would close the control loopin the digester and minimize alkali and lime consumption.

Various methods of measuring effective alkali have been proposed,however most of these measurements have to be corrected for temperatureeffects and interferences by other cations and anions, as well asorganics. Conductivity sensors have been implemented in some mills, andthese give indirect measurement of effective alkali. They may besuitable for on-line measurements of effective alkali in white or greenliquors, however they are not suitable for black liquor due to a highsolids content and the presence of salts from weak organic acids in theblack liquor. On-line measurements of effective alkali in black liquorshave been attempted in a number of ways ranging from on-linecalorimeters to on-line automatic conductimetric titration methods. Noneof those systems is straightforward. Titration methods encountermaintenance problems, thus most mill site measurements still rely onlaboratory standard methods involving precipitating carbonate andphenoxide ions with barium chloride before performing the titration.

The control of continuous digesters is performed by keeping the chip andwhite liquor feeds at preset levels which are determined by the overallproduction rate. Control is performed by adjusting the temperatureprofile of the cook through the H-factor and determining the resultantblow line kappa number. Kappa number is a measure of pulp lignincontent. One disadvantage of this method is that it assumes uniform chipmoisture content and digester temperature. Since the pulp must beanalyzed in the laboratory for lignin content, there is always a delayin controlling the process.

Other methods to analyze organic content have been developed tocorrelate the amount of organics with pulp yield and kappa number.On-line methods however have not been entirely satisfactory. In U.S.Pat. No. 4,743,339 Faix et al proposes a method for determiningeffective alkali in black liquor based upon on-line infrared circularattenuated reflectance measurements. Faix et al also report [TAPPIproceedings, 1989 Wood and Pulping Chemistry Symposium, Raleigh, N.C.]that one is able to measure the consumption of sodium sulfite and theappearance of lignosulfonates during alkaline sulfite anthraquinonemethanol pulping, but the results were not very accurate because ofspectral non-linearities due to the precipitation of dissolvedcompounds. Certain limitations were therefore found with this method.

Michell in TAPPI Journal 73(4) 235, 1990, suggested a similar method forkappa number determination by correlating the increase in the integratedband intensity at a wave number of 1118 cm⁻¹ with decreasing kappanumber. Unfortunately, this region is also prone to interferences fromthe primary and secondary hydroxyl groups in carbohydrates. No attemptswere made by either Michell or Faix et al to evaluate the spectralregion situated between wave numbers of 1800 to 2900 cm⁻¹ for usefulinformation.

To be useful the direct measurement of effective alkali in processliquors must be free of interferences from both inorganic and organiccompounds. Up to now, no infrared spectrophotometric method for directlymeasuring effective alkali in pulping liquors has been developed andimplemented for routine use in pulp and paper manufacturing.

DISCLOSURE OF INVENTION

The present invention provides a direct monitoring and control methodfor the on-line measurement of effective alkali, residual effectivealkali, carbonate, sulfate and thiosulfate concentrations in processliquors for the production of kraft pulp. The control method is on-lineand largely eliminates the need for frequent equipment maintenance,sample pretreatment and the use of chemical reagents. High samplethroughput allows many process streams to be multiplexed to a singleanalyzer.

Samples of process liquors are analyzed by infrared spectrophotometry,the baseline corrected absorbance of the liquor is measured at apredetermined wavelength, the chemical composition of the liquor is thencalculated, the absorbance is correlated with the concentration of theabsorbing compound and this correlation is made by comparing resultspreviously obtained with standard samples. Process samples are alsoanalyzed by standard analytical methods to establish a correlation withthe data obtained by infrared spectrophotometry.

The on-line analytical procedure for residual effective alkali is usedto analyze digester liquor samples from either batch or continuousdigesters. The on-line analytical sensor can also be used to determinesulfate, thiosulfate, carbonate and hydroxide levels in green and whiteliquors. The reduction efficiency in the recovery boiler and thecausticizing efficiency may also be calculated. Thus, the sensor of thepresent invention replaces automatic titrators and conductivity sensorsand gives much needed information on the carbonate, thiosulfate andsulfate levels in process liquors while improving the control of scalingin the multi-effect evaporators. Furthermore, the sensor measures theextent of white and black liquor oxidation by monitoring thiosulfatelevels as well as performing semi-quantitative determination of theextent of cellulose degradation.

In one embodiment the present invention provides a method fordetermining and controlling effective alkali of liquors in a kraft pulpmanufacturing process, comprising the steps of: withdrawing samples of aliquor from the process, subjecting the samples to infraredspectrophotometry at predetermined wave numbers to producepeak-absorbance measurements relative to a background spectrum of water,determining peak absorbance for different alkali concentrations,correlating relationships between the peak-absorbance measurements ofsamples with the peak absorbance for different alkali concentrations todetermine optimum effective alkali in the samples, and controlling atleast one process parameter to obtain optimum effective alkali of theliquor.

BRIEF DESCRIPTION OF DRAWINGS

In drawings which illustrate embodiments of the present invention,

FIG. 1 is a diagrammatic view of a pulp digester complete with sensingand control apparatus according to one embodiment of the presentinvention,

FIG. 2 is a diagrammatic view of the recovery and recausticizingsystems, complete with sensing and control apparatus according toanother embodiment of the present invention,

FIG. 3 is a graph of absorbance versus wave numbers showing the changein band absorbance at a wave number of 1882 cm⁻¹ for three effectivealkali concentrations,

FIG. 4 is a calibration graph of absorbance versus effective alkaliconcentration for fifteen effective alkali concentrations and asulfidity of 25%,

FIG. 5 is a graph of black liquor absorbance versus wave numbers,

FIG. 6 is a graph of kraft pine lignin absorbance versus wave numbers,

FIG. 7 is a graph of glucoisosaccharinic acid absorbance versus wavenumbers,

FIG. 8 is a graph of the evolution of effective alkali concentration ofblack liquor versus the logarithm of the H-factor obtained duringcooking of hardwood and softwood chips,

FIG. 9 is a graph of the evolution of the lignin, saccharinic acid,hemicellulose and effective alkali absorbances versus the H-factorobtained during cooking of softwood chips,

FIG. 10 is a graph of the value of residual effective alkaliconcentration of black liquor samples taken at hourly intervals from atypical mill continuous digester.

MODES FOR CARRYING OUT THE INVENTION

FIG. 1 shows a diagrammatic representation of a continuous type Kamyrdigester and of a control system as embodied by the invention. Thiscontrol system may be used to monitor the EA consumption during theimpregnation stage of a modified continuous cooking pulping operation.Referring to FIG. 1, a digester 10 is shown with makeup white liquorsupplied through line 12. The liquor in the digester 10 is indirectlyheated through a transfer line by high pressure steam supplied through asteam supply line 14 and valve 16. Temperature readings of the cookingliquor are taken by a heat sensor 18 located in steam line 20 andcommunicated to a computer 22. Black liquor is withdrawn from thedigester 10 through the upper extraction screen 23 and then sent throughan upper heater 24 using a recirculating loop 26. A second steam line 28provides steam to a second recirculating loop 30 in which the liquor iswithdrawn from digester 10 through the lower extraction screen 27 andsent to a lower heater 32.

Chips are fed to the digester 10 through line 34. A chip level control36 controls the level in the digester by means of valve 38. Samples fromthe digester are continuously withdrawn from the extraction liquor line40 at withdrawal point 42. For other tests, samples are withdrawn fromthe sample point 44 in the upper heater loop 26 or sample point 46 inthe lower heater loop 30.

The samples are fed through a flow cell 48 for continuous measurement ofthe infrared circular attenuated total reflectance (CATR) of the liquor.Alternatively, a square section attenuated total reflectance cell may beused instead. The CATR cell 48 admits infrared light that is detected bya Fourier transform infrared (FT-IR) spectrometer 50. The spectrometer50 correlates the liquor infrared absorbance at a wave number of about1882 cm⁻¹ with the residual effective alkali of the flowing liquor. Thesignal from the spectrometer 50 is fed to the computer 22 to controleither the cooking duration through the H-factor, which is done byregulating the flow of steam through valve 16 in line 14, or the flow ofwhite liquor through line 12. The extraction liquor flows through line40 to the flash tanks (not shown) on its way to the recovery cycle.

A batch digester is controlled similarly to the continuous digestershown in FIG. 1. Samples are drawn at different digestion times.Modifications may be made by regulating the flow of black liquorrecirculation or adjusting the temperature and the cooking time.Alternatively, the infrared light from the spectrometer may betransmitted by infrared fiber optics. These fibers communicate betweenan instrumentation cubicle and a nearby flow cell through which liquorflows from remote locations.

FIG. 2 shows a diagrammatic view of the recovery and of therecausticizing systems, complete with a sensing and control apparatusembodied by the invention. Black liquor passes through multi-effectevaporators (not shown) and is admitted to the recovery furnace 60 togenerate flue gases 62 and smelt 64. The smelt 64 flows to the smeltdissolving tank 66 to form green liquor. Green liquor samples are takenat sample withdrawal point 68 in line 70 leading to the green liquorclarifier 72. The samples are fed through a small bore conduit 74 into asample stream multiplexer 76 and a flow through CATR cell 78 whichadmits infrared light that is detected by a Fourier transformspectrometer 80. The spectrometer 80 correlates the infrared absorbanceof the liquor at wave numbers of about 1104 cm⁻¹ and 1001 cm⁻¹ with thesulfate and thiosulfate concentrations of the liquor, respectively.Readings from the spectrometer 80 on both concentrations are thentransferred to computer 22 which indirectly calculates an arbitraryvalue for the reduction efficiency by setting a minimum target value forsulfate concentration. The quantity of thiosulfate and sulfate cominginto the tank with the weak wash liquor is relatively minor and does notinterfere with this measurement.

The computer is then adapted to adjust operational variables of therecovery furnace to improve the reduction of sulfate and thiosulfate tosulfide in the lower furnace. Alternatively, this information may bepassed to an operator for manual adjustments. The liquor then proceedsto the green liquor clarifier 72. Dreg deposits are filtered in thedregs filter 82 before passing through the disposal line 84. Greenliquor recovered from the dregs filter 82 flows from the clarifier 72through line 86 and is sampled at sample withdrawal point 88. Thesampled liquor passes through line 90 to the sample stream multiplexer76. It is then analyzed with flow through cell 78 and spectrometer 80.The spectrometer correlates the respective carbonate and hydroxideconcentrations with their corresponding infrared absorbances at wavenumbers of about 1386 cm⁻¹ and 1882 cm⁻¹.

Liquor in line 86 from the clarifier 72 enters the slaker 92 where avariable quantity of calcium oxide is added through valve 94 to formcalcium hydroxide. A series of three or more causticizers 96 allow mostof the sodium carbonate to react with the calcium hydroxide to formsodium hydroxide and calcium carbonate. The resulting suspension thenproceeds to the white liquor clarifier 98. The clarified liquor is thensampled at withdrawal point 100 at the exit of the white liquorclarifier 98. The white liquor is sampled through line 102 to themultiplexer 76 and the flow through cell 78 where it is analyzed forcarbonate and hydroxide by spectrometer 80. The sampling of thedifferent sample withdrawal points 68, 88 and 100 occurs in sequencethrough the multiplexer 76. Lime mud from the clarifier 98 is thenwithdrawn and calcined in a lime kiln (not shown) to form quicklime(CaO). This is then recycled to the slaker 92.

The output data about the liquor collected from sample withdrawal points88 and 100 are used to monitor carbonate and hydroxide levels, and toestimate the causticizing efficiency (CE) given a suitable time delaycorresponding to the duration of the causticizing reaction by computer22. The computer 22 also controls valve 94 to adjust the lime feed rateto the slaker 92.

A combined system of pipes, light guides or infrared fiber optic cablesare used for remote sensing in the recovery and recausticizing systemsto sample liquor from multiple locations and thereby minimize systemcosts by allowing multiple streams to be analyzed by a single FT-IRapparatus.

Experiments were conducted utilizing a Perkin-Elmer [Norwalk, Conn.]1610 FT-IR spectrometer to record all spectra. Spectra recorded were theresults of 64 averaged scans over a spectral range of 4400 cm⁻¹ (2.25micrometers) to 650 cm⁻¹ (15.50 micrometers). The spectra were run atroom temperature using a micro-boat germanium-crystal version of theCIRCLE CELL (T.M.) CATR accessory supplied by Spectra-Tech [Norwalk,Conn.]. Oxidized strong black liquor (BL), diluted to 23% solids, wasused as the source of spent liquor. Three pairs of green (GL) and white(WL) liquor samples, along with the black liquor sample, were analyzedusing both FT-IR and selected TAPPI, CPPA and SCAN standard methods forEA, sulfate and carbonate, respectively. Standard solutions of sodiumhydroxide, sodium carbonate, sodium sulfate and sodium thiosulfate wereprepared (percent w/w) by direct weighing. Solutions of sodium hydroxidewere prepared separately (9.5, 8.5, 7.5, 6.5, 5.5, 4.5, 3.5, 2.5, 1.5,0.5 percent w/w) by direct weighing on a Mettler PJ3000 analyticalbalance with a readability of 0.01 g. Standard solutions (5, 4, 3, 2, 1,0.5 and 0.2 percent w/w) were similarly prepared for sodium carbonate,sodium sulfate and sodium thiosulfate. Higher-concentration solution ofsodium carbonate (10, 9, 8, 7, 6, and 5 percent w/w) were also preparedto simulate the higher levels found in kraft mill green liquors.Concentrations were converted to g/l (as Na₂ O) for further dataanalysis. Standard white liquor solutions (5, 15, 25, 35, 45, 55, 65 g/las Na₂ O; sulfidity: 25%) were also prepared. The transmittance spectraof these solutions were then ratioed against pure water spectra, and theresults transformed to absorbance values. Peak-height absorbances wereread at wave values of about 1001 cm⁻¹ for thiosulfate, about 1104 cm⁻¹for sulfate, about 1386 cm⁻¹ for carbonate, and about 1882 cm⁻¹ forhydroxide. No interferences or infrared peaks due to hydrosulfide orpolysulfide ions were found. A Beer's law calibration plot was thenestablished from peak-absorbance measurements using linear regressionparameters. These are given in Table I for EA, carbonate, sulfate andthiosulfate. Integrated-absorbance calibration curves were alsocalculated for carbonate levels typical of green liquors. Curves weredrawn on paper and the area between a wave number of about 1200 cm⁻¹ wasestimated by cutting out the paper surface. This surface was thenweighted with an analytical balance. This calculation can also routinelybe performed on other commercial systems by integration software. Toverify the precision and accuracy of the CATR method, selected resultsfor EA, REA and dead-load levels for actual mill samples were obtained.For these samples, background corrections to the absorbance wereperformed to account for baseline shifts due to high concentrations ofsodium hydroxide and/or organic acids. Mill results (expressed as g/lNa₂ O) are summarized in Table II, and details are discussed in thefollowing examples.

                                      TABLE I                                     __________________________________________________________________________    Regression parameters                                                                    Effective                                                                            Sodium                                                                              Sodium Sodium                                         REGRESSION Alkali Carbonate                                                                           Sulfate                                                                              Thiosulfate                                    PARAMETER  (as Na.sub.2 O)                                                                      (as Na.sub.2 O)                                                                     (as Na.sub.2 O)                                                                      (as Na.sub.2 O)                                __________________________________________________________________________    Constant   0.0052 0.0171                                                                              0.0007 0.0005                                         Std Err of Y Est                                                                         0.0025 0.0040                                                                              0.0003 0.0005                                         R Squared  0.9973 0.9910                                                                              0.9993 0.9799                                         No. of Observations                                                                      10     10    5      5                                              Degrees of Freedom                                                                        8      8    3      3                                              X Coefficient                                                                            0.0013 0.0044                                                                              0.0066 0.0026                                         Std Err of Coef.                                                                         0.0001 0.0002                                                                              0.0001 0.0002                                         __________________________________________________________________________

                                      TABLE II                                    __________________________________________________________________________    Mill sample results                                                           NaOH (g/l, as Na.sub.2 O)                                                                      Na.sub.2 CO.sub.3 (g/l, as Na.sub.2 O)                                                    Na.sub.2 SO.sub.4 (g/l, as Na.sub.2 O)           SAMPLE                                                                              FT-IR                                                                              ACTUAL.sup.1                                                                        FT-IR ACTUAL.sup.2                                                                        FT-IR                                                                              ACTUAL.sup.3                                __________________________________________________________________________    WL-1  82.2 83.3  22.4  21.3  3.3  3.5                                         GL-1  26.0 26.3  .sup. 76.3.sup.5                                                                    80.0  4.2  4.2                                         WL-2  80.7 81.8  18.3  17.9  7.0  6.7                                         GL-2  26.6 28.3  .sup. 77.4.sup.5                                                                    73.3  6.4  6.2                                         WL-3  81.6 84.6  23.5  22.5  2.9  3.0                                         GL-3  24.4 25.5  .sup. 74.0.sup.5                                                                    74.3  2.4  2.8                                         BL     6.3  6.8  n/a   n/a   .sup. 3.9.sup.6                                                                     4.3.sup.4,6                                __________________________________________________________________________     .sup.1 by CPPA J.12 standard procedure;                                       .sup.2 by SCAN N2 standard procedure                                          .sup.3 by SCAN N3 standard procedure;                                         .sup.4 by TAPPI T624 standard procedure                                       .sup.5 by integrated absorbances;                                             .sup.6 wt % on dry blackliquor solids (23.1%)                            

EXAMPLE 1

The infrared spectrum for three different EA concentrations is seen inFIG. 3. An example of a typical calibration curve is shown in FIG. 4 forwhite liquor. Careful least-squares regression analysis (least-squarescurve fitting) of the data (Table I, Column 1) shows that EA levels areaccurately measured by infrared spectrophotometry in kraft liquors. Thestandard deviation for both the absorbance and the intercept of thecalibration curve shown in FIG. 4 is about 0.00015. Since the slope isabout 0.00072, this translates into a concentration error of 0.4 g/l (asNa₂ O) at the 2σ confidence level. Precision is thus fair, but could beimproved by doing more replicate runs or using a more sensitivedetector. The observed peak at a wave number of about 1882 cm⁻¹ (FIG. 3)is due to sodium hydroxide monohydrate. This hydrate forms when asufficient amount of sodium hydroxide is dissolved in water. The amountof monohydrate present in the solution is thus directly proportional tothe amount of EA. This peak therefore gives an accurate indication ofthe EA levels in the liquors. High levels of sodium hydroxide also causesmall, upward reproducible shifts in the baseline near 1001 cm⁻¹, and1386 cm⁻¹ wave numbers. The baseline shifts represent about 10% of thetotal absorbance. These baseline absorbances can be estimated at awavelength situated near the base of a peak and must be subtracted fromthe peak absorbances readings at the respective carbonate, thiosulfateand sulfate peaks to obtain the absorbance due to the compound. The peaksituated at a wave number of about 1882 cm⁻¹ is easy to detect and doesnot interfere with any other peaks. The information obtained from thispeak can therefore be used to correct for the small, correspondingbaseline shift at other wavelengths, using multilinear regressiontechniques. Sodium hydroxide shows another peak at a wave number ofabout 2736 cm⁻¹. EA results for three white and green liquors (Table II,Column 1) agree with those obtained with standard methods.

EXAMPLE 2

A spectrum of digester black liquor is shown in FIG. 5. The sodiumhydroxide peak at a wave number of about 1882 cm⁻¹ is weak.Nevertheless, this spectral feature is still clearly visible. It wasalso found that the absorbance at a wave number of about 1030 cm⁻¹ couldbe used as a reliable baseline for the sulfate peak to correct for thecombined signal due to lignin and carbohydrate compounds absorbing inthis region. The result for sulfate is corrected by subtracting thevalue of the baseline absorbance at a wave number of about 1030 cm⁻¹from the peak absorbance at a wave number of about 1104 cm⁻¹. Eventhough a sharp peak for lignin can be seen at 1491 cm⁻¹, the other peaksdue to lignin (1299, 1118 and 1030 cm⁻¹) are weak and appear mostly asshoulders. For example, the lignin peak at 1118 cm⁻¹ is small and doesnot interfere with sulfate determination. Although peaks due tohemicellulose (1355 cm⁻¹, shoulder), lignin (1299 cm⁻¹, shoulder),inorganic carbonate (1386 cm⁻¹) and carboxylate ions of organic acids(1403 cm⁻¹) strongly overlap, the maximum of the carbonate peak is stillclearly visible at a wave number of about 1386 cm⁻¹. However, theprecise determination of carbonate in black liquor is difficult becauseof the strong overlap of those peaks. Similarly, the multiple peaks atwave numbers of about 1550 cm⁻¹ represent overlapping bands fromsyringyl lignin (1575 cm⁻¹) and carboxylate groups of organic acids(1545 cm⁻¹) resulting from hemicellulose degradation. The lack ofhydroxyl bands near 3500 cm⁻¹ in FIG. 5 also confirms that these groupsare present as sodium salts in alkaline solutions. Results for EA andsulfate in black liquor (BL) shown in Table II agree with those obtainedfrom standard methods.

EXAMPLE 3

Peak assignments given in Example 2 for lignin were verified bydissolving 60 grams INDULIN AT [Westvaco, Chemical Division, CharlestonHeights, S.C.] kraft pine lignin polymer in one liter of white liquor(EA=35 g/l; sulfidity=25%). The sample spectrum was scanned against awhite liquor background. The CATR spectrum of pure lignin is shown inFIG. 6, with four peaks situated at wave numbers of about 1491, 1295,1115 and 1035 cm⁻¹. The match between these peaks and theiridentification in FIG. 5 is apparent. Similarly, glucose was dissolvedin a caustic solution (EA=75 g/l, sulfidity=0%) and heated to formglucoisosaccharinic acid. The sample spectrum was scanned against acaustic background. The CATR spectrum of glucoisosaccharinic acid isshown in FIG. 7, with intense peaks at a wave number of about 1550 and1400 cm⁻¹ due to carboxylate ions and a broad absorption peak extendingfrom about 1025 to about 1075 cm⁻¹ due to the carbon-oxygen single bondsof primary and secondary hydroxyl groups. The match with the peaksassigned to carboxylate salts of carbohydrate degradation products inFIG. 5 is also apparent.

EXAMPLE 4

The evolution of the effective alkali during a laboratory kraft cook isshown in FIG. 8. The experimental conditions were: alkali charge, 16%(on o.d. wood) for softwood and 13% for hardwood; sulfidity, 25%;liquor-to-wood ratio, 4.5/1.0; time to cooking temperature, 80 minutes;cooking temperature, 170° C.; final H-factor, 1800 (softwood: jack pine)and 1100 (hardwood:aspen). Hardwood cooks are usually performed at alower alkali charge than softwoods because of the lower lignin contentin hardwood. Agreement between titration and infrared measurements isapparent. The effective alkali signal can thus be monitored throughoutthe cook. The sharp drop is EA for the softwood cook is seen at highH-factors, and probably indicates a marked loss in pulp yield. This isshown in FIG. 9 where the relative evolution of the lignin, saccharinicacid and effective alkali absorbances is displayed versus the H-factorobtained during cooking of softwood (jack pine) chips. The increase inthe lignin peak at a wave number of 1491 cm⁻¹ tapers off near the end ofthe cook, indicating that bulk delignification is no longer takingplace. On the other hand, after an initial increase, a decrease in bothsaccharinic acid peaks near the middle of the cook indicates that thereis some decarboxylation of hemicellulose degradation products. This isfollowed by a sudden increase at the end of the cook which probablysuggests that saccharinic acids originating from cellulose are nowaccumulating in the liquor.

EXAMPLE 5

The change in residual effective alkali (REA) at hourly intervals in atypical full-scale mill digester is shown in FIG. 10. The coefficient ofvariation for this type of measurement is 4.5%. Comparison withtitrimetric methods are very favourable.

From the above examples it can be seen that different liquors in thekraft pulping process can be analyzed and the effective alkali ofliquors can be controlled by correlating the relationship betweenpeak-absorbance measurements of samples with the peak absorbance fordifferent alkali concentrations and thus determine an optimum effectivealkali in the sample. Consequently the process can be controlled toobtain the optimum effect alkali of the liquor by varying at least oneprocess variable.

Various changes may be made to the embodiments shown herein withoutdeparting from the scope of the present invention which is limited onlyby the following claims.

The embodiments of the present invention in which an exclusive propertyor privilege is claimed are defined as follows:
 1. A method forcontrolling the operation of a recovery boiler in the preparation ofkraft pulp wherein smelt is produced in a recovery furnace and fed to asmelt dissolving tank to form green liquor, which comprises the stepsof:withdrawing samples of a green liquor after the smelt dissolvingtank; subjecting the samples to infrared spectrophotometry at a wavenumber of about 1100 cm⁻¹ for sulfate and at a wave number of about 1000cm⁻¹ for thiosulfate to produce peak-absorbance measurements relative toa background spectrum of water; subjecting samples of sulfate andthiosulfate solutions of different known concentrations to infraredspectrophotometry at the same wave numbers in order to establish sulfateand thiosulfate calibration curves; correlating relationships betweenthe peak-absorbance measurements of the samples with the peakabsorbances of the samples of sulfate and thiosulfate solutions todetermine optimum sulfate and thiosulfate concentrations in the samples;and adjusting the recovery boiler operation as required to bring thesulfate and thiosulfate concentrations in the green liquor from theconcentration thereof in the withdrawn samples, as determined by thepeak-absorbance measurement thereof, to optimum concentration.
 2. Themethod for controlling the operation of a recovery boiler according toclaim 1 wherein the samples of green liquor are subjected to infraredspectrophotometry at a wave number of about 1030 cm⁻¹ to producebaseline-absorbance measurements and wherein baseline-absorbancemeasurements for different sulfate and thiosulfate concentrations aredetermined.
 3. The method according to claim 1 wherein thespectrophotometry is performed in attenuated total reflectance.
 4. Themethod according to claim 3 wherein a circular section attenuated totalreflectance cell is used.
 5. The method according to claim 3 in which asquare section attenuated total reflectance cell is used.
 6. The methodaccording to claim 1 wherein the spectrophotometry is performed in aflow through cell for continuous measurements.
 7. The method accordingto claim 1 wherein the relationships between the peak-absorbancemeasurements of samples with the peak absorbance for different alkaliconcentrations is obtained using least squares curve fitting.